1. Field of the Invention
The present invention is in the technical field of illumination devices. More particularly, the present invention is in the technical field of multicolor illumination devices and projection systems using the same.
2. Description of the Related Art
Narrow band light sources, such as laser diodes (LDs) and light emitting diodes (LEDs), emit light with a narrow wavelength-band. Such narrow band lights are typically highly saturated in color, which can produce much more vivid colors than those usually seen in nature. Wide band light sources, such as those employing phosphor materials excited by an excitation light, emit lights with wide wavelength-bands. Such wide band light are typically less saturated in color, which can produce natural and soft color similar to those usually seen in nature. Such wide band light also has higher color rendering index, which is a measure of a light's ability to show natural colors of illuminated objects. The closer a white color is to sun light in its spectrum, the higher its color rendering index. Sun light has a continuous and relatively smooth spectrum which covers the entire visible range.
In some applications, such as stage lighting and decorative lighting, both narrow band light and wide band light are desired. For example, when it is desired to achieve high color contrast and vivid lights, narrow band light sources can be used. When white light is used, it is often desired to employ light with high color rendering index so that the illuminated objects appear to be more natural. Currently, lamps which emit white light are usually used together with color filters to achieve a color light. However, it is very inefficient to generate highly saturated color using lamps that emit white light. Also, such a method can only achieve a limited number of color lights, and it is difficult to achieve a continuous transition from color to color.
So solve the above problems, a known multi-channel illumination device can be provided, as shown in FIG. 1. In the illumination device shown in FIG. 1, lights emitted from a green LED array 1, a red LED array 2 and a blue LED array 3 are collimated by collimators 6, and then combined by an X-shape dichroic filter set 5 into one beam. The combined light is projected by an optical system 8 onto a screen 9. In this device, red, green and blue lights are used as primary lights to produce other colored light or white light. For example, a yellow light can be obtained by a mixture of red light and green light. A white light can be obtained by a mixture of red, blue and green lights, the spectrum 111 of which is shown in FIG. 1a. The white light spectrum 111 contains a blue peak 111a, a green peak 111b and a red peak 111c. By independently controlling the intensities of the three primary colors (R, G, B) lights, it is possible to obtain every color light with chromaticity coordinates located inside the color gamut constructed by the three primary colors (R, G, B) and the illumination efficiency is improved as compared to the use of color filters. However, the mixed color light and mixed white light often look unnatural and their color renderings are often unsatisfactory. The problem comes from the fact that the three highly saturated primary lights have narrow spectra, and the spectrum of the mixed light has peaks and valleys, which is very different from the spectrum of sun light 110. As shown in FIG. 1a, the spectrum of RGB LED mixed white light has two valleys around 480 nm and 590 nm, making it difficult to show cyan and amber colors.
To solve the problem of low color rendering index, FIG. 2 shows the optical structure of another known illumination device, where RGB LEDs 151, 152, 153 are used as primary color light sources, and white light emitted from a set of white LED sources 25 are combined with the primary color light bundle by lens 31 into a light collector 4. The white source 25 is phosphor-converted source (i.e. a source that generates light using phosphor materials excited by an excitation light) and mixes yellow light from the phosphor with unabsorbed blue excitation light to generate a white light, which has a wide wavelength-band as shown in FIG. 2a. The light emitted from this illumination device looks more natural and has a higher color rendering index compared to the spectrum of FIG. 1a because of the addition of wide band white light. However, because of the large spectrum overlap between white light and the RGB primary color lights, the white light cannot be combined with the primary color lights by the dichroic filter set 154 as it would lead to heavy losses, and a light combiner that combines light in the spatial domain (e.g. lens 31) is needed. As a result, the size of the mixed light bundle will increase as compared to the size of the primary light from the RGB LEDs, which will result in an increase of etendue and decrease of brightness of the combined light.
It should be noted that for a low-saturation single color light to look more natural, its spectrum needs to be relatively wide so that it is more comparable to the spectrum of natural light having low color saturation. Then, the low fidelity of a low-saturation light and the low color rendering index of a white light are the same problem at their base, which is the narrow spectrum width of the light. Thus, it should be understood that when the following descriptions refer to the low color rendering index of a white light, it also refers to the low fidelity of a low-saturation light.
As seen from the above discussion, the use of wide band white light in FIG. 2 can solve the problem of low color rendering index, but tends to reduce the brightness of the light source. There is a tradeoff between high color rendering index and high brightness.